1. Field of the Invention
The present invention relates to a mask producing method, an image outputting device and computer readable recording medium, in particular, to a mask producing method for producing a mask of a threshold matrix by which a halftone-dot image is created with an image quality improved, the threshold matrix being used for generation of halftone image in digital image processing, and, to a so-called FM mask, a type of the threshold matrix, attracting attention in terms both of processing speed and image quality, an image outputting device, and a computer readable recording medium storing a software program, and mask data, thereof.
2. Description of the Related Art
As a method of producing a halftone image in digital image processing, i.e., a method of expressing a shade or gray scale of an image by a combination of on-dot (for example, black dot) and off-dot (for example, white dot), there are “dithering” and “error dispersion method”, and they have the following characteristics, respectively.
First, dithering will be described now.
Dithering is a method of converting original multi-level image data into a corresponding dot pattern using a threshold matrix (referred to as a “dither matrix”, or, simply, “mask”, hereinafter) produced based on a specific rule, by comparing with the original multi-level image data every pixel.
FIG. 1 shows a type of dithering. As shown in the figure, dithering expresses a gray scale by changing the density of on-dots, and includes a frequency modulation scheme (FM mask) dithering and an area modulation scheme (AM mask) dithering. According to the FM mask dithering, the density of uniformly dispersed on-dots is controlled.
There, in order to reproduce 8-bit-per-pixel multi-level information by an outputting unit/device having a lower expressing capability, on-dot density should express information in depth direction (8 bits) of the original image data has, on plane coordinates. In this case, in order to keep one-to-one correspondence between the characteristics, such as on-dot (for example, black dot) density and gray scale of the output image, the number of on-dots present within a predetermined area should be determined for every gray scale level, necessarily, as shown in FIG. 2A, for example.
The above-mentioned dither matrix is produced as a result of on-dot arrangements being summarized in a table on a predetermined area for respective gray scale levels, while matching is held with regard to the order of on-dot occurrence along gray scale before and after a relevant gray scale level, as shown in FIG. 2B. FIG. 2A shows on-dot arrangement determined for each gray scale level, while FIG. 2B shows a process of producing a dither matrix according to the on-dot arrangements shown in FIG. 2A.
The control of the order of on-dot occurrence along the gray scale is made for preventing that a difference in dot pattern in the meantime may be recognized in an output image as a stepwise manner, at a time a dither matrix to be applied is switched on an image in which gray scale changes gradually.
Although it is possible as a dither matrix to apply a matrix having a size the same as that of an input original image theoretically, since a memory capacity needed would become huge, it is common to apply a dither matrix of size much smaller than an original image, while repeatedly shifting it a position, area by area, in a manner of tile placement, on an original image, as shown in FIG. 3, so as to perform conversion to a halftone dot pattern.
Since this dithering is comparatively simple for operation processing, the processing speed can be secured, and, this scheme may be applied to an image requiring a medium (not very high) image quality. In this scheme, generally, it is noted that a measure should be taken against Moiré.
On the other hand, although processing speed is slow due to complex processing required, the error dispersion method may be applied to an image requiring a high or very high image quality. Of course, this scheme requires relatively high cost on the other hand.
Thus, since any schemes have particular merits and demerits, as for dithering and error dispersion schemes, it is common to properly select one of them according to a nature of an image output uniting applied or a type of image to be processed.
At late in the 1900s, a halftone processing scheme having features including merits of these two types of schemes was proposed. Generally, this scheme is called “FM mask method”, “FM screen method” or “blue noise mask method”, and is similar to a Bayer-type method which is a type of “dithering”, and employs a way of the same frequency modulation type (FM) mask comparing method.
According to this. FM mask method, a threshold matrix is applied which is determined in consideration of frequency characteristic (blue noise characteristic, or BN characteristic) only concerning high frequency component except low frequency component in case of threshold determination. Though it is a mask comparing method, it is possible to prevent periodicity in low frequency from being conspicuous as in case of Bayer-type method or dot-collection type dithering, and, also, to prevent generation of Moire, as a result, the resolution characteristic as superior as that in case of application of the error diffusion method can be acquired. Therefore, this method attracts attentions from various fields, i.e., a printing field, and so forth, and various approaches are tried thereon.
For example, as an initial trial, after carrying out Fourier transform on a completely random (white noise) dot pattern, then filtering with a filter with BN characteristic is performed, and, then, inverse Fourier transform is performed, so that an ideal FM mask may be produced, according to “Method and Device for converting Gray Scale Image into Halftone Image by using Blue Noise Mask” disclosed in Japanese patent No. 2622429.
Moreover, according to “The Void-and-Cluster Method for Dither Array Generation” (Robert Ulchney, Digital Equipment Corporation, Maynard, Mass., 01754-2571), SPIE, vol. 1913, the contents thereof being incorporated therein by reference, a method (referred to as a ‘void and cluster method or scheme’, hereinafter) is disclosed of comparing a void (image region having coarse on-dots) and a cluster (image region having dense on-dots), and exchanging on-dots therebetween, thereby, performing mask optimization.
However, according to any of these schemes, as a calculation result finally obtained depends on a random pattern as a starting point, it should be different each time. Furthermore, since it is necessary to reconstruct a uniform distribution state from an uneven distribution state, huge calculation time is required if the size of a mask becomes large.
Then, according to “Improved method and device of generating halftone image while reducing worms in gray scale image” disclosed by Japanese laid-open patent application No. 8-80641, a starting mask (initial dot pattern) made according to the error diffusion scheme is applied.
According to this scheme, a random element can be eliminated as a result of a dot pattern used as a starting point being arbitrarily controllable, and thereby, recursion of a calculation result can be secured. Furthermore, calculation time can be shortened while the quality of the mask finally produced can be improved as a result of a uniform distribution being able to be applied at an initial stage. However, as this method is a so-called ‘sequential method’ in which, from such a starting dot pattern, optimization is performed step by step along gray scale, a dot arrangement is thus necessarily restricted by an immediately precedingly determined dot pattern.
Accordingly, even when the starting dot pattern is ideal in terms of avoiding graininess and/or texture characteristic, it is a feature being held only at a starting point, and, after adding on-dots thereto so as to produce a dot pattern in a subsequent gray scale level, this feature may be degraded. Then, after repeating this process for producing dot patterns for the subsequent gray scale levels, so as to build a final FM mask, difference from the starting point is accumulated gradually, and, accordingly, the final FM mask may be far different from the ideal one.
Although various optimization functions and search methods for ideal dot arrangement have been tried in order to prevent such quality degradation in dot arrangement as described above, any schemes may not completely solve this problem as long as the method is the above-mentioned sequential method by which, as mentioned above, restriction due to immediately preceding gray scale level should be accumulated.